1. Field of the Invention
The present invention relates to the movement of documents, for purposes of capturing an image thereof, and particularly to the detection of the presence of overlapping sheets on a transport. More specifically, this invention is directed to a sensor for detecting the presence of multiple layers of sheet material, paper bearing printed indicia for example, and especially to an acoustic device for discriminating between single and plural moving thin sheets which are in surface contact. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Related Art
In the processing of documents, for example for image capture pursuant to "machine reading", economic considerations dictate that the indicia bearing thin sheets be serially delivered to an image capture station at a high document through-put rate. The documents to be processed are customarily stacked in a magazine and individual sheets are extracted from the stack and delivered to a transport such as a vacuum conveyor. As a result of ambient operating conditions and phenomena such as electrostatic attraction and friction, multiple documents are sometimes substantially simultaneously extracted from a magazine and deposited on the transport in partial overlapping relationship or in registration with one another. The most common type of such a delivery failure is a double document feed. If the multiple fed documents are in registration, the image of the lowermost document in the viewing direction will not be captured and, accordingly, important data will not be read and processed. If the multiple fed documents are partially overlapped, the end result is that the image of neither document will be captured, a feed failure will likely result and operator intervention will be required. Obviously, the former condition, i.e., registration of multiple documents, is the more insidious problem since it is less likely of the two feed failure modes to be noticed.
Various attempts have been made to detect the presence of multiple, i.e., overlapping, documents on a paper transport. The prior approaches to solving this long standing problem have embodied optical and capacitive measurement techniques. The prior approaches, however, have been too slow and/or lacking in sensitivity, unable to operate with invariance to sheet thickness or combinations of thickness and/or unable to operate with invariance to print color or print density.